1. Field of the Invention
The present invention relates to a display device with a touch sensor. The present invention further relates to a method of operating a display device with a touch sensor.
2. Related Art
WO 2009075577 discloses a display device comprising a flexible display panel that includes a display structure responsive to output signals from the data processing facility, and a capacitive touch sensitive structure. In an embodiment the display structure and the touch sensitive structure share a transparent common electrode layer that is alternately coupled to a display driver and to a touch screen driver via a switching element.
During a first phase of operation the transparent common electrode layer, such as an ITO layer, is coupled to the display driver, so that the display driver can update the content of the display panel.
During a second phase of operation the common electrode layer is coupled to the touch screen driver. In the second phase of operation the transparent common electrode layer is driven with an AC voltage signal. Typically the transparent electrode layer is driven at each of four corners by a respective driver, each providing an AC-voltage signal (e.g. sinusoidal or block-shaped signal waveform), with the same amplitude, frequency and phase. By measuring the balance between the AC currents of all four corners, the coordinates of a touching finger can be determined. The finger causes a capacitive load of the transparent common electrode layer, thereby changing the AC currents of the four corners. The coordinates of the location where the panel is touched can then be calculated from the relative values of the AC-currents measured at the corners, for example, using a lookup table (LUT).
A shared common electrode layer is advantageous, in that it is not necessary to add extra layers to provide the touch screen functionality. In this way the display panel having the touch screen functionality can be relatively thin as compared to the case where the touch sensitive structure does not share the common electrode layer with the display structure. This is particularly advantageous for a flexible display, as a thinner display panel allows for a smaller radius of curvature. Also, there is no deterioration of optical display properties (e.g. brightness), as would have been caused by an extra layer.
However, it has been found that a touch sensitive structure that shares the common electrode layer with the display structure requires relatively high drive voltages as compared to embodiments wherein the common electrode layer is not shared in order to achieve a sufficient sensitivity. The requirement for relatively high driving voltages results in a relatively high power consumption. Moreover relatively powerful drivers are necessary to generate the driving voltages.